1. Field of the Invention
The present invention relates to a transparent polycrystalline alumina tube. In more detail, the present invention relates to a transparent polycrystalline alumina tube and a high pressure metal vapour lamp using the tube.
2. Description of the Prior Art
A high pressure metal vapour lamp, such as a high pressure sodium lamp has become very popular by a reason of its excellent characteristics of the high luminance efficiency. As the discharge tube of such a high pressure metal vapour lamp, generally a transparent polycrystalline alumina tube has been used.
The transparent alumina had been disclosed, for instance, in Philippe D.S. St. Pierre et al U.S. Pat. No. 3,026,177 issued Mar. 20, 1962 and entitled "Process for producing transparent polycrystalline alumina"; Robert L. Coble U.S. Pat. No. 3,026,210 issued Mar. 20, 1962 and entitled "Transparent alumina and method of preparation"; Kazuo Kobayashi et al U.S. Pat. No. 3,792,142 issued Feb. 12, 1974 and entitled "Utilizing mixtures of yttria, magnesia, and lanthanum oxide in manufacture of transparent alumina".
When the transparent polycrystalline alumina tube is used as such discharge tube of a high pressure metal vapour lamp, both ends of the tube are sealed, such as, for instance, disclosed in William C. Louden U.S. Pat. No. 3,448,319 issued June 3, 1969 entitled "Niobium end seal". The sealing is effected by firing and due to this firing process a certain number of tubes cause cracks which made the tubes unsuitable for use. A cause of such low yield of the tube is considered to be a recent tendency to make the crystal grain size of the polycrystalline alumina tube substantially larger than that disclosed in the Kobayashi et al U.S. Pat. No. 3,792,142 under an object of improving the luminance efficiency by increasing the translucency of the tube. Generally, in a translucent polycrystalline alumina, the translucency gradually increases according to an increase of crystal grain size, but the mechanical strength decreases according to an increase of crystal grain size.
The conventional high pressure metal vapour lamp using a transparent polycrystalline alumina tube having larger crystal grain size as the discharge tube has drawbacks in that it tends to cause cracks at the end portion of the tube or the portion near the electrode due to sudden temperature change of the sealed metal vapour at the time of switching on and off.
From the above explanation it may be understood that two mutually inconsistent requirements, namely an excellent translucency or good transmission property and a high mechanical strength or a high thermal shock resistance, are imposed for the transparent polycrystalline alumina tube used as the discharge tube of a high pressure metal vapour lamp.